Wireless communication system

ABSTRACT

A wireless communication system includes a wireless base station, a mobile station configured to communicate with the wireless base station, and a relay station configured to relay the communication between the wireless base station and the mobile station. When the number of the mobile stations communicating with the wireless base station is judged to have become smaller than or equal to a given number, a power controller in the relay station executes at least one of power control to cut off the power for relaying the communication, power control to perform intermittent operation, and power control to perform power-saving operation.

This is a continuation of International Application PCT/JP2009/051440,filed on Jan. 29, 2009, now pending, the contents of which are hereinwholly incorporated by reference.

FIELD

The present invention relates to wireless communication systems whichinclude, for example, a mobile communication system and a wireless LAN(Local Area Network).

BACKGROUND

In recent years, new high-speed communication service called LTE (LongTerm Evolution) has been expected to become standards for thecommunications of mobile stations such as mobile phones, andLTE-Advanced systems, which are an improved version of LTE systems, havebeen under discussion at the 3GPP (3rd Generation Partnership Project).

LTE-Advanced systems are intended also as IMT (International MobileTelecommunication)-Advanced systems, which are an improved version ofIMT-2000 systems and have been decided to be given consideration at theITU-R (International Telecommunication Union Radio communicationssector).

Typical examples of IMT-2000 systems are W-CDMA (Wideband-Code DivisionMultiple Access), CDMA one, and WiMax (Worldwide Interoperability forMicrowave Access).

For LTE-Advanced systems, consideration is being given, based on LTEsystems, to widening of uplink and downlink bandwidths, uplink MIMO(Multiple Input Multiple Output), MBMS (Multimedia Broadcast MulticastService), and introduction of a relay station (repeater station) betweena wireless base station and a mobile station.

SUMMARY

According to an aspect of the invention, a wireless communication systemincludes: a wireless base station; mobile stations each configured tocommunicate with the wireless base station; and a relay stationconfigured to relay communication between the wireless base station andthe mobile stations, wherein the relay station includes a powercontroller which is configured to execute at least one of power controlto cut off power for relaying the communication, power control toperform intermittent operation, and power control to performpower-saving operation if it is judged that a number of the mobilestations communicating with the wireless base station has become smallerthan or equal to a given number.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary configuration of a wirelesscommunication system.

FIG. 2 illustrates an exemplary configuration of a wireless basestation.

FIG. 3 illustrates how a service area is expanded.

FIG. 4 illustrates how a dead area is dealt with.

FIG. 5 illustrates an exemplary configuration of a relay station.

FIG. 6 illustrates an exemplary configuration of a mobile station.

FIG. 7 is a diagram explaining power control.

FIG. 8 is a diagram explaining the power control.

FIG. 9 illustrates an operation sequence for the power control.

FIG. 10 illustrates an exemplary configuration of a wireless basestation.

FIG. 11 illustrates an exemplary configuration of a relay station.

FIG. 12 illustrates an exemplary configuration of a mobile station.

FIG. 13 illustrates an overall operation sequence including handover.

FIG. 14 illustrates an exemplary configuration of a relay station.

FIG. 15 exemplifies radio resources allocated during normalcommunication.

FIG. 16 exemplifies radio resources allocated at the time of bandlimitation.

FIG. 17 exemplifies radio resources allocated at the time of bandlimitation (no mobile stations connected).

FIG. 18 illustrates an operation sequence for the transition frompower-controlled operation to normally powered operation.

FIG. 19 illustrates an operation sequence for the transition from thepower-controlled operation to the normally powered operation.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below withreference to the accompanying drawings. FIG. 1 illustrates an exemplaryconfiguration of a wireless communication system. The wirelesscommunication system 1 includes a wireless base station 10, a mobilestation 30 configured to communicate with the wireless base station 10,and a relay station 20 configured to relay the communication between thewireless base station 10 and the mobile station 30.

The relay station 20 includes a relay communicator 2 a and a powercontroller 2 b. The relay communicator 2 a controls the transmission andreception of the relay communication between the wireless base station10 and the mobile station 30. The power controller 2 b is aware of thenumber of the mobile stations 30 communicating with the wireless basestation 10, and if it is judged that the number of the communicatingmobile stations 30 has become smaller than or equal to a given number,the power controller 2 b executes at least one of power control to cutoff the power for relaying the communication, power control to performintermittent operation, and power control to perform power-savingoperation.

The manner of how the wireless base station 10, the relay station 20 andthe mobile station 30 are configured will be now described. FIG. 2illustrates an exemplary configuration of the wireless base station. Thewireless base station 10-1 includes an antenna al, an uplink wirelessunit 11, a power control notification information extractor 12, aconnection request signal extractor 13, a radio link controller 14, aradio link control signal generator 15, a power control notificationinformation generator 16 (corresponding to a timing signal notifier),and a downlink wireless unit 17.

The uplink wireless unit 11 down-converts the radio signal received fromthe relay station 20 or the mobile station 30 via the antenna al andoutputs the received data. When power control notification information(e.g., information to the effect that the power control is to beexecuted at the relay station 20 or information on power control starttiming) is received from the relay station 20, the power controlnotification information extractor 12 extracts the power controlnotification information from the received data. When a connectionrequest signal is received from the mobile station 30 or the relaystation 20, the connection request signal extractor 13 extracts theconnection request signal from the received data.

The radio link controller 14 performs radio link control in accordancewith the power control notification information, the connection requestsignal from the mobile station 30 or the relay station 20, and aconnection request signal of its own. The radio link control signalgenerator 15 generates a radio link control signal and multiplexes thegenerated signal and transmit data.

The power control notification information generator generates powercontrol notification information (e.g., information on the power controlstart timing for causing the relay station 20 to execute the powercontrol) and superimposes the generated information on the transmitsignal. The downlink wireless unit 17 up-converts the transmit data andtransmits the converted data via the antenna al.

The relay station will be explained below. The relay station is providedbetween a wireless base station and a mobile station with a view toexpanding the service area (cell extension) of the wireless base stationor to dealing with a dead area (dead spot), for example.

FIG. 3 illustrates how a service area is expanded. A mobile station 120is located outside the cell 100 a of a wireless base station 100, and arelay station 110 is installed inside the cell 100 a. The relay station110 has a relay area 110 a covered thereby, and the mobile station 120is located inside the relay area 110 a.

If the relay station 110 does not exist, the mobile station 120 isunable to communicate with the wireless base station 100 because it islocated outside the cell 100 a. However, if the relay station 110 isinstalled as illustrated and also if the mobile station 120 existswithin the relay area 110 a of the relay station 110, the wireless basestation 100 and the mobile station 120 can communicate with each otherbecause the wireless communication is relayed by the relay station 110,even though the mobile station 120 is located outside the cell 100 a.

FIG. 4 illustrates how a dead area is dealt with. The relay station 110is installed inside the cell 100 a of the wireless base station 100. Letit be assumed that a dead area 110 b exists within the cell 100 a andthat the mobile station 120 is located inside the dead area 110 b. Therelay area 110 a of the relay station 110 covers the dead area 110 b.

Without the relay station 110, it is difficult for the mobile station120 to communicate with the wireless base station 100 since the mobilestation 120 exists within the dead area 110 b. However, if the relaystation 110 is installed as illustrated and also if the relay area 110 aof the relay station 110 covers the dead area 110 b, the wireless basestation 100 and the mobile station 120 can communicate with each otherbecause the wireless communication is relayed by the relay station 110,even though the mobile station 120 is located within the dead area 110b.

As conventional wireless techniques, a technique has been proposed inwhich a receiving circuit for receiving wide- and medium-band signals ispowered off during the standby time, and is powered on when a call isreceived, thereby saving the battery power (cf. Japanese Laid-openPatent Publication No. 08-275243 (paragraph nos. [0161] to [0174], FIG.12)).

The relay station has a small service area because of its purpose ofintroduction. Thus, compared with the service area of an ordinarywireless base station, the number of mobile stations that communicatevia the relay station is thought to be small. Also, in the middle of thenight and early in the morning, the number of mobile stations thatcommunicate via the relay station generally decreases, and possibly,even a situation could occur where no mobile station communicates viathe relay station.

Thus, since the relay station is occasionally used by only a smallnumber of mobile stations (or in some cases by no mobile stations), aproblem arises in that electric power is consumed wastefully if therelay station is continuously operated.

It is therefore necessary that power control for a power supply of therelay station (power on-off control or intermittent operation control)be executed at the relay station. If the relay station executes suchpower control on its own judgment, however, the wireless base stationand the mobile station lose their party of communication without beingnotified in advance, with the result that a fault (e.g., connectionfailure) occurs. Further, if the relay station executes the powercontrol on its own judgment, the resources used by the relay stationuntil then become unavailable.

FIG. 5 illustrates an exemplary configuration of the relay station. Therelay station 20-1 includes antennas a2 and a3, an uplink transmitter21-1, a downlink receiver 21-2, an uplink connection request signalextractor 22 a (corresponding to a signal extractor), a downlinkconnection request signal extractor 22 b, a radio link controller 23-1,a radio link control signal generator 23-2, a power control notificationinformation extractor 24, a received power measurement unit 25, a powercontrol executor 26, a power control notification information generator27, an uplink receiver 28-1, and a downlink transmitter 28-2. The powercontrol notification information extractor 24, the received powermeasurement unit 25, the power control executor 26 and the power controlnotification information generator 27 are the functions included in thepower controller 2 b illustrated in FIG. 1.

The downlink receiver 21-2 receives via the antenna a2 the radio signaltransmitted from the wireless base station 10, then down-converts thereceived signal, and outputs downlink data. If a downlink connectionrequest signal is included in the downlink data, the downlink connectionrequest signal extractor 22 b extracts the downlink connection requestsignal from the downlink data.

The radio link controller 23-1 performs downlink radio link control inaccordance with the downlink connection request signal. The radio linkcontrol signal generator 23-2 generates a downlink radio link controlsignal and superimposes the generated signal on the downlink data.

When the power control notification information is received from thewireless base station 10, the power control notification informationextractor 24 extracts the power control notification information fromthe downlink data and sets power control timing with respect to thepower control executor 26. The downlink transmitter 28-2 up-converts thedownlink data, on which the radio link control signal and the powercontrol notification information generated by the power controlnotification information generator 27 are superimposed, and transmitsthe converted data to the mobile station 30 via the antenna a3.

The uplink receiver 28-1 receives via the antenna a3 the radio signaltransmitted from the mobile station 30, then down-converts the receivedsignal, and outputs uplink data. The received power measurement unit 25measures the received power from the uplink data. If an uplinkconnection request signal is included in the uplink data, the uplinkconnection request signal extractor 22 a extracts the uplink connectionrequest signal from the uplink data.

The radio link controller 23-1 performs uplink radio link control inaccordance with the uplink connection request signal. The radio linkcontrol signal generator 23-2 generates an uplink radio link controlsignal and superimposes the generated signal on the uplink data. Theuplink transmitter 21-1 up-converts the uplink data, on which the radiolink control signal and the power control notification informationgenerated by the power control notification information generator 27 aresuperimposed, and transmits the uplink data to the wireless base station10 via the antenna a2.

A given value of mobile station number threshold as well as a givenvalue of received power threshold are externally set with respect to thepower control executor 26, and in accordance with the uplink anddownlink connection request signals, the received power and theexternally set thresholds, the power control executor 26 executes thepower control (details of the power control will be described later).The elements on which the power control is performed are indicated bydashed arrows in the figure and include, for example, the uplinktransmitter 21-1, the downlink receiver 21-2, the uplink receiver 28-1,the downlink transmitter 28-2, the radio link control signal generator23-2, the uplink connection request signal extractor 22 a, the downlinkconnection request signal extractor 22 b, the radio link controller23-1, the power control notification information extractor 24, thereceived power measurement unit 25, and the power control notificationinformation generator 27.

FIG. 6 illustrates an exemplary configuration of the mobile station. Themobile station 30-1 includes an antenna a4, a downlink wireless unit31-1, an uplink wireless unit 31-2, a received power measurement unit32, a power control notification information extractor 33, a linkconnection controller 34, and a connection request signal generator 35.

The downlink wireless unit 31-1 down-converts the radio signal receivedfrom the wireless base station 10 or the relay station 20 via theantenna a4, and outputs received data. The received power measurementunit 32 measures the power of the received data. If the power controlnotification information is included in the received data, the powercontrol notification information extractor 33 extracts the power controlnotification information from the received data. The link connectioncontroller 34 performs link connection control in accordance with thereceived power and the power control notification information. Theconnection request signal generator 35 generates an uplink connectionrequest signal and superimposes the generated signal on transmit data.The uplink wireless unit 31-2 up-converts the transmit data andtransmits the converted data via the antenna a4.

The following describes the manner of how the number of mobile stationsis calculated at the relay station 20. When mobile stations 30 arecommunicating with the wireless base station 10 via the relay station20, the relay station 20 may confirm the number of the mobile stationsbeing relayed thereby by any one of the three methods described below,for example.

1. The relay station 20 measures the received power (or SINR (Signal toInterference plus Noise Ratio)) from each mobile station 30 and comparesthe measured received power with the received power threshold. If themeasured received power is greater than the received power threshold,the relay station 20 regards the corresponding mobile station as amobile station being relayed thereby. If the measured received power issmaller than the received power threshold, the relay station 20 regardsthe corresponding mobile station as a mobile station not relayedthereby. Thus, the relay station 20 can recognize the number of themobile stations being relayed thereby.

The received power may be measured by using any one of RF (RadioFrequency) signal, IF (Intermediate Frequency) signal, demodulatedsignal, and decoded signal. Also, the received power may be measured byusing the Pilot signal (dedicated Pilot or Sounding Pilot) transmittedfrom the mobile station 30. The received power from the mobile station30 is measured by the received power measurement unit 25 in FIG. 5, andthe number of mobile stations is calculated in the power controlexecutor 26 by comparing the measured received power with the receivedpower threshold.

2. Where the connection request signal or a control signal (e.g.,scheduling request) is received from the mobile station 30 via a randomaccess channel, for example, the relay station 20 regards that mobilestation 30 as a mobile station being relayed thereby. If such signal isnot received from the mobile station 30 over a fixed period of time, therelay station 20 regards that mobile station 30 as a mobile station notrelayed thereby. Thus, the relay station 20 can recognize the number ofthe mobile stations being relayed thereby.

The connection request signal or control signal from the mobile station30 is extracted by the uplink connection request signal extractor 22 ain FIG. 5, and the number of mobile stations is calculated by the powercontrol executor 26.

3. The relay station 20 determines whether the connection request signalor control signal (e.g., scheduling request) transmitted from the mobilestation 30 via the random access channel, for example, is directedthereto or to another relay station. If it is judged that the signal isdirected to the own relay station 20, the corresponding mobile station30 is regarded as a mobile station being relayed by the own relaystation 20. If it is judged that the signal is directed to a differentrelay station, the corresponding mobile station 30 is regarded as amobile station not relayed by the own relay station. Accordingly, therelay station 20 can recognize the number of the mobile stations beingrelayed thereby.

Whether the connection request signal or control signal from the mobilestation 30 is directed to the own relay station or not is determined bythe uplink connection request signal extractor 22 a in FIG. 5, and thenumber of mobile stations is calculated by the power control executor26.

In this manner, the relay station 20 keeps track of the number of themobile stations being relayed thereby, and compares the number of mobilestations with the threshold value for the number of connected mobilestations (mobile station number threshold), which has been notified fromthe wireless base station 10 or has been set in advance. If thethreshold is not exceeded, the relay station 20 executes the powercontrol. For the mobile station number threshold, “0” may be selected(the number of mobile stations is zero).

The power control executed in the relay station 20 will be nowexplained. FIGS. 7 and 8 are diagrams explaining the power control,wherein FIG. 7 illustrates radio links between the relay station 20 andthe wireless base station 10 and between the relay station 20 and themobile station 30, and FIG. 8 exemplifies power control patterns inrelation to the dedicated radio links.

Where one of the following three control actions, namely, power-on,power-off, and intermittent operation, is carried out as the powercontrol on the dedicated radio links r1 to r4 illustrated in FIG. 7,there are a total of 63 (=4³−1) combinations of the control actions,excluding the normal operation in which all radio links are powered on(FIG. 8 illustrates four typical patterns mostly used among thecombinations). In the following, the power control patterns (1) to (4)for the relay station 20, illustrated in FIG. 8, will be explained.

(1) The transmitting and receiving systems (radio links r1 and r2)associated with the wireless base station 10 and the transmitting andreceiving systems (radio links r3 and r4) associated with the mobilestation 30 are all powered off (i.e., the relay station is in itsentirety powered off).

(2) The transmitting and receiving systems (radio links r1 and r2)associated with the wireless base station 10 are powered off, and thetransmitting and receiving systems (radio links r3 and r4) associatedwith the mobile station 30 are intermittently operated.

(3) The transmitting and receiving systems (radio links r1 and r2)associated with the wireless base station 10 are powered off, thereceiving system (radio link r3) associated with the mobile station 30is intermittently operated, and the transmitting system (radio link r4)associated with the mobile station 30 is powered on.

(4) The transmitting system (radio link r1) associated with the wirelessbase station 10 is powered off, the receiving system (radio link r2)associated with the wireless base station 10 is intermittently operated,the receiving system (radio link r3) associated with the mobile station30 is powered off, and the transmitting system (radio link r4)associated with the mobile station 30 is powered on.

An operation sequence for the power control according to the firstembodiment will be now described. FIG. 9 illustrates the power controloperation sequence, wherein the mobile station number threshold is setto “0”.

S1: The power controller 2 b compares the number of connected mobilestations with the threshold to determine whether or not the formerexceeds the latter.

S2: If the threshold is not exceeded (if the number of mobile station issmaller than or equal to the given number), the relay station 20notifies the wireless base station 10 and the mobile stations 30 thatthe relay station 20 will be powered off.

S3: On receiving the notification, the wireless base station 10determines the timing for powering off the relay station 20 and notifiesthe relay station 20 of the start timing for initiating the powercontrol (power-off timing).

S4: When notified of the power-off timing, the relay station 20 notifiesthe other wireless base stations, the mobile stations connected to therelay station 20, and the mobile stations in standby mode of thepower-off timing.

S5: The power supply to the relay station 20 is turned off in accordancewith the power-off timing.

S6: Where the power supply to the relay station 20 has been turned off,the wireless base station 10 reallocates the radio resources so that theuplink (mobile station 30→relay station 20) and downlink (relay station20→mobile station 30) radio resources used for the communication by therelay station 20 until then may be used by the wireless base station 10or other relay stations.

In this manner, the power control of the relay station 20 can beperformed, thus making it possible to reduce the power consumption.Generally, in the relay station 20 and the mobile station 30, thetransmitting amplifier in particular consumes much power, so that theconsumption of power by the transmitter accounts for a large part of thetotal power consumption. Since the power supply to the transmitter canbe turned off, the power consumption can be significantly reduced.

Also, the radio resources that become unused because of the power-offare allocated to other relay stations or the wireless base station,whereby the radio resources can be reused to enable efficienttransmission.

In the foregoing description, the relay station 20 is assumed to executeDF (Decode and Forward relaying wherein the received radio signal isrelayed by performing thereon processes such as demodulation, errorcorrection decoding, re-encoding, and modulation). The power control canbe similarly performed with respect to an AF relay station which isconfigured to execute AF (Amplify and Forward relaying wherein thereceived radio signal is merely amplified for relaying and is notdecoded) for the data transmission and which is capable of transmittingand receiving the control signal at all times.

Further, in the above exemplary case, it is the relay station 20 thatcounts the number of mobile stations to execute the power control. Sincethe wireless base station 10 can identify the mobile stations connectedthereto, the wireless communication system may be configured such thatthe wireless base station 10 counts the number of mobile stations andnotifies the relay station 20 of the number of mobile stations (thewireless base station 10 can control the radio links between thewireless base station 10 and the relay station 20 and also can directlyor indirectly control the radio links between the relay station 20 andthe mobile station 30, and therefore, the wireless base station 10 iscapable of counting the number of mobile stations).

A second embodiment will be now described. In the second embodiment,when the power control of the relay station 20 is to be performed,handover is executed with respect to the mobile stations 30communicating via the relay station 20 and the wireless base station 10.

First, the manner of how the wireless base station, the relay stationand the mobile station are configured will be explained. In thefollowing description, like reference signs refer to like elementsalready explained, and description of such elements is omitted. Only thedifferences will be described below.

FIG. 10 illustrates an exemplary configuration of the wireless basestation. The wireless base station 10-2 includes the antenna al, theuplink wireless unit 11, the power control notification informationextractor 12, the connection request signal extractor 13, the radio linkcontroller 14, the radio link control signal generator 15, the powercontrol notification information generator 16, the downlink wirelessunit 17, a handover request signal extractor 18 a, and a handovercontroller 18 b. Thus, the wireless base station 10-2 additionallyincludes the handover request signal extractor 18 a and the handovercontroller 18 b.

If a handover request signal is included in the received data, thehandover request signal extractor 18 a extracts the handover requestsignal from the received data. In accordance with the handover requestsignal, the handover controller 18 b executes handover control.

FIG. 11 illustrates an exemplary configuration of the relay station. Therelay station 20-2 includes the antennas a2 and a3, the uplinktransmitter 21-1, the downlink receiver 21-2, the uplink connectionrequest signal extractor 22 a, the downlink connection request signalextractor 22 b, the radio link controller 23-1, the radio link controlsignal generator 23-2, the power control notification informationextractor 24, the received power measurement unit 25, the power controlexecutor 26, the power control notification information generator 27,the uplink receiver 28-1, the downlink transmitter 28-2, and a handoverrequest unit 29.

Thus, the handover request unit 29 is additionally provided. When thepower control is to be executed, the handover request unit 29 generatesa handover request signal and superimposes the generated signal on thedownlink and uplink data.

FIG. 12 illustrates an exemplary configuration of the mobile station.The mobile station 30-2 includes the antenna a4, the downlink wirelessunit 31-1, the uplink wireless unit 31-2, the received power measurementunit 32, the power control notification information extractor 33, thelink connection controller 34, the connection request signal generator35, a handover request signal extractor 36 a, a handover controller 36b, and a handover control signal generator 36 c. Thus, the handoverrequest signal extractor 36 a, the handover controller 36 b and thehandover control signal generator 36 c are the newly added elements.

If a handover request signal is included in the received data, thehandover request signal extractor 36 a extracts the handover requestsignal from the received data. In accordance with the handover requestsignal, the handover controller 36 b executes handover control. Thehandover control signal generator 36 c generates a handover controlsignal and superimposes the generated signal on the transmit data.

FIG. 13 illustrates an overall operation sequence including handover.

S11: The relay station 20 confirms the number of mobile stations 30connected thereto.

S12: The relay station 20 notifies the wireless base station 10 that thepower control will be executed.

S13: The mobile station 30, the relay station 20 and the wireless basestation 10 execute handover.

S14: The wireless base station 10 notifies the relay station 20 of powercontrol timing.

S15: The relay station 20 executes the power control in accordance withthe notified power control timing.

Details of the operation will be now explained. The number of mobilestations is compared with the threshold by the power controller 2 b ofthe relay station 20, and if it is judged that the threshold is notexceeded, the power controller 2 b notifies the wireless base station 10and the mobile stations 30 that the relay station 20 will be poweredoff. Further, the power controller 2 b requests of one or more mobilestations 30 then communicating via the relay station 20 and the wirelessbase station 10 that the mobile stations be handed over to other relaystation or wireless base station.

On receiving the handover request, the dedicated mobile stations 30measure the received powers from other relay stations or wireless basestations to select a target of handover, and then executes handover.When the handover of all mobile stations 30 that received the handoverrequest is completed, the relay station 20 again notifies the wirelessbase station 10 that the relay station 20 will be powered off.

When the notification is received, the wireless base station 10determines the timing at which the relay station 20 is to be poweredoff, and notifies the relay station 20 of the power-off timing. Onreceiving the notification of the power-off timing, the relay station 20notifies the mobile stations connected to other wireless base stationsor to the relay station as well as the mobile stations in the standbymode of the power-off timing. Subsequently, the relay station 20 ispowered off at the power-off timing.

Where the relay station 20 has been powered off, the wireless basestation 10 reallocates the radio resources so that the uplink (mobilestation 30→relay station 20) and downlink (relay station 20→mobilestation 30) radio resources used for the communication by the relaystation 20 until then can be used by the wireless base station or otherrelay stations.

In this manner, the power control of the relay station is executed, sothat the power consumption can be reduced. Also, since the radioresources that become unused because of the power-off are allocated toother relay stations or the wireless base station, the radio resourcescan be reused to achieve efficient transmission.

Further, by carrying out a similar process with respect to wireless basestations and other relay stations, it is possible to cause mobilestations to concentrate at a certain wireless base station or relaystation. This enables reduction of the power consumption of the wholesystem.

A third embodiment will be now described. In the first and secondembodiments, the relay station 20 sends the power control request to thewireless base station 10, and on receiving the request, the wirelessbase station 10 notifies the relay station 20 of the power controltiming. In the third embodiment, on the other hand, the relay station 20itself determines the power control timing and notifies the wirelessbase station 10 or the mobile stations 30 of the determined timing.

The number of the mobile stations is compared with the threshold by thepower controller 2 b, and if it is judged that the number of mobilestation is smaller than or equal to the threshold, the power controller2 b determines the timing for executing the power control and notifiesthe wireless base station 10 or the mobile stations 30 that the relaystation 20 will be powered off at the determined power-off timing. Also,the power controller 2 b requests of one or more mobile stations 30 thencommunicating via the relay station 20 and the wireless base station 10that the mobile stations be handed over to other relay station orwireless base station. The subsequent operation is identical with thatof the second embodiment.

A fourth embodiment will be now described. In the aforementioned powercontrol of the relay station 20, the power supply to the relay station20 is turned off. According to the fourth embodiment, the transmitterand receiver of the relay station 20 associated with the wireless basestation 10 are powered off while the transmitter and receiver associatedwith the mobile station 30 are intermittently operated.

FIG. 14 illustrates an exemplary configuration of the relay station. Therelay station 20-4 includes the antennas a2 and a3, the uplinktransmitter 21-1, the downlink receiver 21-2, the uplink connectionrequest signal extractor 22 a, the downlink connection request signalextractor 22 b, the radio link controller 23-1, the radio link controlsignal generator 23-2, the power control notification informationextractor 24, the received power measurement unit 25, the power controlexecutor 26, the power control notification information generator 27,the uplink receiver 28-1, the downlink transmitter 28-2, an a transmitpower controller 2-1.

Thus, the transmit power controller 2-1 is additionally provided. Duringthe execution of the power control, the transmit power controller 2-1operates the downlink transmitter 28-2 while reducing the power suppliedthereto.

The following describes the operation of the fourth embodiment (controloperation whereby the transmitter and receiver associated with thewireless base station 10 are powered off while the transmitter andreceiver associated with the mobile station 30 are intermittentlyoperated). The transmission of data from the relay station 20 to themobile station 30 is stopped. Further, the transmission of commoncontrol CH (channels) (or common control signals) to the mobile station30 is switched to intermittent transmission. Control signals (controlCH) necessary for the mobile station 30 to connect to the relay station20 include, for example, a broadcast channel for transmittinginformation (e.g., used frequency) about the service area (cell) of therelay station 20, a control channel for transmitting a pilot signal(reference signal) necessary for cell selection, and a synchronizationchannel for synchronizing between the relay station and the mobilestation.

By intermittently transmitting these channels, it is possible to reducethe power consumption. The power amplifier (PA) of the downlinktransmitter 28-2, in particular, consumes much power, and therefore,transmitting as little data as possible serves to reduce the powerconsumption.

Further, the relay station 20 stops receiving the transmit data from themobile station 30. Also, the relay station 20 receives, on anintermittent basis, the random access channel (RACH) containing aconnection request from the mobile station 30, the control signalnecessary for the connection, such as pilot, and the control CH. Bycausing the relay station 20 to operate in this manner, it is possibleto execute the power control, whereby the power consumption can bereduced.

A fifth embodiment will be now described. In the fifth embodiment, thetransmitter and receiver of the relay station 20 associated with thewireless base station 10 are powered off, the transmitter associatedwith the mobile station is operated with reduced power, and the receiverassociated with the mobile station 30 is intermittently operated.

When the number of mobile stations relayed by the relay station 20 issmaller than or equal to the threshold, the relay station 20 executesthe power control in cooperation with the wireless base station 10 andthe mobile station 30. Also, to ensure that after the power control isinitiated, the relay station 20 can resume the normal operation onreceiving a connection request from the mobile station 30, the relaystation is allocated specific radio resources less than the radioresources necessary for normal communication.

Specifically, to ensure connection with the mobile station 30, the relaystation 20 requests the wireless base station 10 to allocate radioresources whereby necessary control signals can be transmitted to themobile station 30 (though, in actuality, no such control signals may betransmitted). As exemplary radio resources, FIG. 15 illustratesexemplary radio resources allocated during the normal communication,FIG. 16 illustrates exemplary radio resources allocated at the time ofband limitation, and FIG. 17 illustrates exemplary radio resourcesallocated at the time of band limitation (no mobile stations connected).

The control signals (control CH) necessary for the mobile station toconnect to the relay station 20 include, for example, a broadcastchannel for transmitting information (e.g., used frequency) about theservice area (cell) of the relay station 20, a control channel fortransmitting the pilot signal (reference signal) necessary for cellselection, and a synchronization channel for synchronizing between therelay station 20 and the mobile station 30.

Generally, these control CH are transmitted from the relay station 20 atall times and not on an intermittent basis. Since the transmit poweraccounts for a large part of the power consumption of the relay station20, however, the transmit power needs to be suppressed in order toreduce the power consumption of the relay station 20. Thus, somemeasures need to be taken to suppress the transmit power in order toreduce the power consumption.

On the other hand, the power control of the relay station 20 is executedwhen the traffic between the relay station and the mobile stations 30 issmall, and accordingly, the number of mobile stations that newlyoriginate a call is naturally small. Thus, even if the transmit powerfor transmitting the aforementioned control signals and control CH isreduced and a virtual service area (cell) narrows as a result, only alimited number of mobile stations fails in connection due to thereduction of the transmit power.

The transmit power for transmitting the control signals and the controlCH is reduced, for example, to ½ of the normal transmit power.Specifically, the number of mobile stations and the threshold arecompared with each other by the power controller 2 b, and if it isjudged that the number of mobile stations is smaller than the threshold,the power controller 2 b turns off the power supply to the transmitterand receiver associated with the wireless base station 10, and alsorequests the downlink transmitter 28-2 associated with the mobilestations 30 to reduce the transmit power, so that the transmit power ofthe downlink transmitter 28-2 is reduced. Also, the power controller 2 bnotifies the downlink transmitter 28-2 that only the control signals areto be transmitted, so that the downlink transmitter 28-2 is controlledso as to transmit the control signals only.

Further, the power controller 2 b performs the power control on theuplink receiver 28-1 such that the uplink receiver 28-1 receives signalson an intermittent basis. The uplink receiver 28-1 may be intermittentlyoperated at intervals equal to the wireless TTI (Transmission TimeInterval) or in the time unit of msec. or the like, for example. Also,the intervals of the intermittent operation may be varied in accordancewith estimated traffic. By controlling the relay station in theaforementioned manner, it is possible to efficiently reduce the powerconsumption of the relay station 20.

Transition from the power-controlled operation to normally poweredoperation will be now described with reference to a sequence diagram.FIG. 18 illustrates an operation sequence for the transition from thepower-controlled operation to the normally powered operation.

S21: On receiving the control signal from the relay station 20, themobile station 30 measures the received power and compares the measuredreceived power with the received powers from other relay stations orother wireless base stations to select a relay station 20 to connect to.

S22: Where the relay station 20 executing the power control is selected,the mobile station 30 transmits, to the relay station 20, a connectionrequest based on the control signal received from the relay station 20.It is assumed that the radio resources are allocated beforehand so thatthe connection request can be transmitted from the mobile station 30 tothe wireless base station 10 via the control channel, the random accesschannel or the like.

S23: Prior to the start of communication with the mobile station 30, therelay station 20 notifies the wireless base station 10 that theoperation will be switched to normally powered operation.

S24: The wireless base station 10 transmits to the relay station 20information indicating power recovery timing.

S25: The relay station 20 terminates the power control in accordancewith the power recovery timing and switches to the normally poweredoperation. Specifically, the relay station 20 powers on the transmittingand receiving systems associated with the wireless base station 10,restores to the normal transmit power the transmit power fortransmitting the control signals to the mobile station 30, and controlsthe transmitter so that data can be transmitted therefrom.

S26: Connection control is executed between the wireless base station 10and the relay station 20 (the relay station 20 requests the wirelessbase station 10 to reallocate the radio resources or to increase theallocation).

S27: Connection control is executed between the mobile station 30 andthe relay station 20.

S28: After the normally powered operation is resumed, the mobile station30, the relay station 20 and the wireless base station 10 executehandover.

The aforementioned control makes it possible to quickly switch theoperation of the relay station from the power-controlled operation backto the normally powered operation. After the normally powered operationis resumed, the mobile station 30 is handed over to the relay station 20then normally operating with the power normally supplied, so that themobile station 30 is able to communicate with the wireless base station10 via the relay station 20. After the relay station 20 resumes thenormally powered operation, a handover execution request may betransmitted to execute the handover.

A sixth embodiment will be now described. In the fifth embodiment, whena connection request is received from the mobile station 30, theoperation of the relay station is switched from the power-controlledoperation to the normally powered operation to recover the power supply.According to the sixth embodiment, the relay station 20 resumes thenormally powered operation upon receiving a request from the wirelessbase station 10.

The wireless base station 10 transmits such a request when, for example,the number of mobile stations connected to a certain wireless basestation increases to such an extent that the operation of the relaystation 20 needs to be switched to the normally powered operation inorder to distribute the load. Also, when the transmissioncharacteristics between the mobile station 30 and the wireless basestation 10 need to be improved and the need can be met by connecting therelay station 20, the relay station 20 is requested to resume thenormally powered operation.

FIG. 19 illustrates an operation sequence for the transition from thepower-controlled operation to the normally powered operation. It isassumed that the mobile station 30 is currently connected to andcommunicating with the wireless base station 10 and that the relaystation 20 operating in the power control operation exists near themobile station 30 and is transmitting the control signal necessary forconnection.

S31: The mobile station 30 starts measuring the received powers for cellselection. Specifically, the mobile station 30 measures the receivedpower from the wireless base station 10 to which the mobile station 30is currently connected, the received power from the relay station 20,and the received powers from other wireless base stations. Let ussuppose that among the measured received powers, the received power fromthe relay station 20 is found to be the strongest.

S32: The mobile station 30 notifies the wireless base station 10 of theselected cell (in this case, the relay station 20).

S33: On receiving the notification, the wireless base station 10requests the relay station 20 to switch the operation from thepower-controlled operation to the normally powered operation(notification of power recovery timing), and also reallocates the radioresources.

S34: When the request for the transition from the power-controlledoperation to the normally powered operation is received, the relaystation 20 powers on the transmitter associated with the wireless basestation 10 as well as the receiver associated with the mobile station 30and, using the radio resources allocated by the wireless base station10, sets up an uplink to the wireless base station 10 and an uplink fromthe mobile station 30. Setting up the uplink from the mobile station 30allows the mobile station 30 to transmit a connection request to therelay station 20.

S35: Connection control is executed between the wireless base station 10and the relay station 20.

S36: Connection control is executed between the mobile station 30 andthe relay station 20.

S37: After the relay station 20 resumes the normally powered operation,the mobile station 30 is handed over to the relay station 20 which isthen normally operating with the power normally supplied. Consequently,the mobile station 30 can communicate with the wireless base station 10via the relay station 20. After the relay station 20 resumes thenormally powered operation, a handover execution request may betransmitted to execute the handover.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatvarious changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1. A wireless communication system comprising: a wireless base station;mobile stations each configured to communicate with the wireless basestation; and a relay station configured to relay communication betweenthe wireless base station and the mobile stations, wherein the relaystation includes a power controller which is configured to execute atleast one of power control to cut off power for relaying thecommunication, power control to perform intermittent operation, andpower control to perform power-saving operation if it is judged that anumber of the mobile stations communicating with the wireless basestation has become smaller than or equal to a given number.
 2. Thewireless communication system according to claim 1, wherein the powercontroller recognizes the number of the relayed mobile stations bymeasuring received powers from the respective mobile stations, comparingthe measured received powers with a received power threshold, regardingthe mobile stations of which the received power is greater than thereceived power threshold, as mobile stations being relayed by the relaystation, and regarding the mobile stations of which the received poweris smaller than the received power threshold, as mobile stations notrelayed by the relay station.
 3. The wireless communication systemaccording to claim 1, wherein: the relay station includes a signalextractor configured to receive and extract connection request signalsor control signals transmitted from the respective mobile stations, andwhere the connection request signal or the control signal is extracted,the power controller regards the mobile station which is transmittingthe connection request signal or the control signal, as the mobilestation being relayed by the relay station, and regards the mobilestation which is not transmitting the connection request signal or thecontrol signal, as the mobile station not relayed by the relay station,to recognize the number of the relayed mobile stations.
 4. The wirelesscommunication system according to claim 1, wherein: the relay stationincludes a signal extractor configured to receive and extract connectionrequest signals or control signals transmitted from the respectivemobile stations, the signal extractor determines whether the connectionrequest signal or the control signal extracted thereby is directed tothe own relay station or other relay station, and where the connectionrequest signal or the control signal is judged to be directed to the ownrelay station, the power controller regards the mobile station which istransmitting the connection request signal or the control signaldirected to the own relay station, as the mobile station being relayedby the relay station, and where the connection request signal or thecontrol signal is judged to be directed to other relay station, thepower controller regards the mobile station which is transmitting theconnection request signal or the control signal directed to the otherrelay station, as the mobile station not relayed by the relay station,thereby recognizing the number of the relayed mobile stations.
 5. Thewireless communication system according to claim 1, wherein: thewireless base station includes a timing signal notifier configured tonotify a timing signal indicating timing for transition from normallypowered operation to the power control, and where the power control isto be executed, the power controller notifies the wireless base stationor the mobile stations that the power control will be executed, thetiming signal notifier of the wireless base station transmits the timingsignal to the relay station when notified of execution of the powercontrol, and the power controller executes the power control inaccordance with the timing signal.
 6. The wireless communication systemaccording to claim 1, wherein, when the power control is executed at therelay station, the wireless base station reallocates radio resources ofuplink from the mobile stations to the relay station and radio resourcesof downlink from the relay station to the mobile stations.
 7. Thewireless communication system according to claim 1, wherein: the relaystation includes a handover request unit, and when the power control isto be executed by the power controller, the handover request unit causesthe mobile stations being relayed or the wireless base station toexecute handover to other relay station or other wireless base station,and after the handover is executed, the power controller executes thepower control.
 8. The wireless communication system according to claim1, wherein: the relay station includes a handover request unit, and thepower controller determines timing for executing the power control, thehandover request unit transmits information about the timing to themobile stations or the wireless base station to cause the mobilestations being relayed or the wireless base station to execute handoverto other relay station or other wireless base station, and after thehandover is executed, the power controller executes the power control.9. The wireless communication system according to claim 1, wherein thepower controller executes the power control by powering off atransmitter and a receiver which relay the communication with thewireless base station, and intermittently operating a transmitter and areceiver which relay the communication with the mobile stations.
 10. Thewireless communication system according to claim 1, wherein the powercontroller executes the power control by powering off a transmitter anda receiver which relay the communication with the wireless basesstation, operating, with reduced power, a transmitter which relays thecommunication with the mobile stations, and intermittently operating areceiver which relays the communication with the mobile stations. 11.The wireless communication system according to claim 1, wherein, if thenumber of the mobile stations has become larger than the given number orif a connection request is received from the wireless base station orthe mobile stations, the power controller terminates the power controland performs normally powered operation.
 12. The wireless communicationsystem according to claim 1, wherein: the relay station includes ahandover request unit, and when the power control is to be executed bythe power controller, the handover request unit transmits a handoverrequest to the mobile stations being relayed or the wireless basestation, the mobile stations and the wireless base station executehandover to other relay station or other wireless base station uponreceiving the handover request, and after the handover is executed, thepower controller executes the power control.
 13. A wirelesscommunication method, comprising: causing a relay station to recognize anumber of mobile stations communicating with a wireless base station;and causing the relay station to execute at least one of power controlto cut off power for relaying the communication, power control toperform intermittent operation, and power control to performpower-saving operation if it is judged that the number of the mobilestations has become smaller than or equal to a given number.
 14. A relaydevice comprising: a relay communicator configured to controltransmission and reception of communication relayed between a wirelessbase station and mobile stations; and a power controller configured torecognize a number of the mobile stations communicating with thewireless base station and execute, if it is judged that the number ofthe mobile stations has become smaller than or equal to a given number,at least one of power control to cut off power for relaying thecommunication, power control to perform intermittent operation, andpower control to perform power-saving operation.